Anode catalyst for fuel cells with polymer electrolyte membranes

ABSTRACT

A platinum/ruthenium alloy catalyst that includes finely dispersed alloy particles on a powdery, electrically conductive carrier material. The catalyst is particularly resistant to carbon monoxide poisoning when the alloy particles display mean crystallite sizes of 0.5 to less than 2 nm.

INTRODUCTION AND BACKGROUND

The present invention relates to a platinum/ruthenium alloy catalystcontaining finely dispersed alloy particles on a powdery, electricallyconductive carrier material. The catalyst is particularly suitable as ananode catalyst for fuel cells having a polymer electrolyte membrane.

Fuel cells are, in principle, gas-operated batteries in which the energyderived from the reaction of hydrogen and oxygen is directly convertedinto electrical energy. The instant invention describes the preparationof catalysts for fuel cells, in particular the preparation of supportedcatalysts based on platinum and platinum alloys for PEM fuel cells(PEM=polymer electrolyte membrane). This type of fuel cell is gaininggrowing importance as a source of current for motor vehicles driven byelectric motors because of its high energy density and robustness.

Compared to conventional combustion engines, fuel cells display very lowemissions with, at the same time, very high efficiency. When hydrogen isused as the fuel gas, water is the only emission formed on the cathodeside of the cell. Motor vehicles with this type of drive are termed ZEV(Zero Emission Vehicles).

Hydrogen is, however, too expensive at the present time and causesproblems in storage and in the fuelling of vehicles. This explains thegrowing importance of the alternative of generating hydrogen directly onboard the vehicle through the reforming of methanol. The methanol storedin the vehicle tank is converted in a steam reforming process at200-300° C. into a hydrogen-rich fuel gas with carbon dioxide and carbonmonoxide as secondary constituents. After converting the carbon monoxideby the shift reaction, preferential oxidation (PROX) or otherpurification process, this fuel gas is conducted directly to the anodeside of the PEM fuel cell. The reformed gas theoretically consists of 75vol. % hydrogen and 25 vol. % carbon dioxide. In practice, however, thisgas still contains nitrogen, oxygen and, depending on degree of purity,fluctuating amounts of carbon monoxide (up to 1 vol. %).

Catalysts based on platinum and platinum alloys are used as catalysts onthe anode side and on the cathode side of the PEM fuel cell. Theseconsist of finely divided precious metal particles that are precipitatedonto a conductive carrier material (generally carbon black or graphite).The precious metal content ranges from 10 to 50 wt. %, based on thetotal weight of the catalyst.

Since conventional platinum catalysts are very sensitive to carbonmonoxide poisoning, the carbon monoxide content of the fuel gas must belowered to below 10 ppm to prevent performance loss in the fuel cellsdue to poisoning of the anode catalyst. This applies in particular tothe PEM fuel cell that is particularly sensitive to carbon monoxidepoisoning with its low working temperatures of 70 to 100° C.

The instant invention relates to the preparation of supported catalystson the basis of bimetallic platinum/ruthenium alloy catalysts thatdisplay a high resistance to carbon monoxide poisoning. Carbon monoxidecontents of the reformed gas in excess of 100 ppm should be possible andlead to virtually no perceptible performance losses in the PEM fuelcell.

The use of novel catalysts of this kind on the anode side of the PEMfuel cell can reduce the number of process steps needed to remove carbonmonoxide from the fuel gas. This leads to a substantial reduction in thesystem costs, to an improvement in the efficiency of the system andmakes the entire system smaller. The new catalysts are therefore ofgreat importance for the introduction of the PEM fuel cell in motorvehicles.

The problem of the poisoning of platinum catalysts by carbon monoxidehas been known for a long time. In view of its special molecularstructure, carbon monoxide is adsorbed onto the surface of the platinum,thereby blocking access for the hydrogen molecules of the fuel gas tothe catalytically active centers of the platinum.

By adding water, the adsorbed carbon monoxide can be oxidized to carbondioxide and can then be removed from the surface of the catalyst. It isalso known that the tolerance of the platinum catalyst to carbonmonoxide poisoning can be improved by alloying or doping the platinumwith other metals.

EP 0 501 930 B1 describes for example quaternary alloys of platinum,nickel, cobalt and manganese as anode catalyst of phosphoric acid fuelcells (PAFC: phosphoric acid fuel cell) that possesses good resistanceto carbon monoxide at the high operating temperatures of a phosphoricacid fuel cell of 160 to 200° C. The size of the alloy particles is inthe region of 3 nm. At the high operating temperatures of the phosphoricacid fuel cell, however, there is at the outset a reduced tendency forthe carbon monoxide to adsorb onto metal surfaces than at the lowoperating temperatures of a PEM fuel cell.

L. W. Niedrach et.al. (J. Electrochemical Techn. 5, 1967, S.318)describe the use of Pt/Ru catalysts as CO-tolerant anode catalysts forsulphuric acid fuel cells. These materials consist of fine Pt/Ru alloypowders with high specific surfaces. They are prepared using theso-called ADAMS process in a melt of platinum chloride, ruthenium andsodium nitrate at 500° C. Because of the high temperatures duringpreparation, these catalysts are present as Pt/Ru alloys. The materialsare not fixed to a carrier and therefore do not constitute supportedcatalysts. Moreover there is no information on their use in a PEM fuelcell.

EP 0 549 543 B1 describes a process for the preparation of supportedcatalysts that contain highly dispersed metal particles with meanparticle sizes of under 2 nm. The process consists in reducing metalions in a suspension of the carrier material by means of a reducingagent in the presence of carbon monoxide and simultaneouslyprecipitating them onto the carrier. The carbon monoxide present isadsorbed onto the precipitating metal particles and thereby hinderfurther particle growth. Following completed precipitation, the catalystis washed and dried at a temperature below 100° C. in a reducingatmosphere. The carbon monoxide is thereby desorbed. Example 4 describesthe preparation of a platinum/ruthenium catalyst on carbon with a meanparticle size of the platinum/ruthenium particles of 1.7 nm. In thiscase, however, the catalyst is not an alloy catalyst, since theadsorption of the carbon monoxide on the metal particles duringprecipitation prevents the formation of an alloy. Nor is any alloyformed during the subsequent temperature treatment up to 100° C. Nostatement is made regarding the properties of this catalyst in use asanode catalyst in a PEM fuel cell with a reformed gas containing carbonmonoxide.

A platinum/ruthenium alloy catalyst on a carrier material has beencommercially available for some time. This is a Pt/Ru alloy catalystwith a precious metal loading between 5 and 40 wt. % and a Pt/Ru atomicratio of 1:1. This catalyst displays a uniform alloy phase that can bedetermined using XRD. Examinations of this catalyst indicated anunsatisfactory tolerance to carbon monoxide, in particular at carbonmonoxide concentrations over 100 ppm and residual oxygen in the fuelgas.

In a recent paper, M. Iwase and S. Kawatsu report on the development ofCO-tolerant anode catalysts (M. Iwase and S. Kawatsu, ElectrochemicalSociety Proceedings, Volume 95-23, S. 12). In this paper, the bestresults were achieved with a Pt/Ru alloy catalyst in which the formationof the alloy was obtained by a special temperature treatment. Thevoltage drop at a current density of 0.4 A/cm² was nonetheless still ca.200 mV at a CO-content of 100 ppm. This is still too high for practicaluse. Still poorer results were, however, achieved with an unalloyedPt/Ru catalyst.

The positive effect of the ruthenium on the resistance to poisoning isattributed to the fact that, in the presence of oxygen when ruthenium ispresent, carbon monoxide is oxidized to carbon dioxide which displays alesser tendency to adsorption on metal surfaces than does carbonmonoxide.

It is therefore an object of the instant invention to prepare aplatinum/ruthenium alloy catalyst on a carrier that displays improvedtolerance to carbon monoxide.

A further object of the instant invention is to provide a catalystsuitable for operation with carbon monoxide-, nitrogen- andoxygen-containing fuel gases which also displays a voltage drop that isas low as possible with high current densities at carbon monoxidecontents of the fuel gas of more than 100 ppm.

SUMMARY OF THE INVENTION

The above and other objects of the invention can be achieved by aplatinum/ruthenium alloy catalyst that contains finely dispersed alloyparticles on a powdery, electrically conductive carrier material. Thealloy catalyst is characterized in that the alloy particles display meancrystallite sizes from 0.5 to less than 2 nm.

It has now surprisingly been found that the Pt/Ru alloy catalystsdisplay a very good Co tolerance up to a concentration of 150 ppm carbonmonoxide in the fuel gas when the alloy particles are smaller than 2 nm.However, the electrochemical activity of the catalysts is diminishedbelow a particle size of 0.5 nm, with the result that smaller particlesare not interesting for use in catalysts.

Another feature of the instant invention is a process for making thecatalyst as described herein.

Yet another feature of the instant invention is the fuel cell containingthe catalyst of the invention.

Still another feature of the invention is the gas diffusion electrodefor the anode side of a PEM fuel cell comprising the catalyst asdescribed herein.

DETAILED DESCRIPTION OF INVENTION

The causes for the improved carbon monoxide tolerance of the alloycatalysts of the invention is not yet fully understood. A possibleexplanation could lie in the fact that the alloy particles displaydifferent surface areas. There are, for example, surface areas that aresubstantially formed by platinum atoms and that are therefore subject tocarbon monoxide poisoning. In addition, there are other surface areasthat are formed by ruthenium atoms. By means of diffusion processes ontothe alloy particles the carbon monoxide adsorbed onto the platinum atomscomes into contact with the ruthenium atoms and can be oxidized there tocarbon dioxide in the presence of oxygen. Because of the small size ofthe alloy particles of the catalyst of the invention, this diffusionprocess also already occurs sufficiently frequently at the low operatingtemperatures of a PEM fuel cell, with the result that an effectiveregeneration of the platinum surfaces is possible.

The speed-determining step of the oxidation of carbon monoxide is thusnot the reaction with the oxygen on the ruthenium surface, but thediffusion of the carbon monoxide on the crystallite surface fromplatinum atoms to ruthenium atoms.

Carbon black, graphited carbon black, graphite or active charcoal withspecific surfaces (BET surfaces, measured in accordance with DIN 66132)of about 40 to 1500 m² /g can be used as electrically conductive carriermaterial for the catalyst. The platinum/ruthenium alloy particles areapplied to these carriers in a concentration of 10 to 50 wt. % relatedto the total weight of the catalyst. The platinum/ruthenium atomic ratiolies between 1:4 and 4:1, with a ratio of 1:1 being, however, preferred.

To prepare the catalyst of the invention the alloy particles must beapplied to the carrier in highly dispersed form with mean particle sizesunder 2 nm. These low particle sizes are only obtained with difficultyusing conventional impregnation procedures. In addition, it is necessaryto ensure that a genuine alloy formation occurs. This is customarilyachieved by temperature treatment at temperatures above 800 to 900° C.These high temperatures lead, however, to an enlargement of theprecipitated metal particles and therefore make it impossible to obtainthe catalyst of the invention.

In accordance with the invention, the precipitation of the preciousmetals onto the carrier material occurs through impregnation with theaid of pre-formed, surfactant-stabilized platinum/ruthenium alloycolloids. The preparation of bimetallic alloy colloids is described inEP 423 627 B1 and in DE 44 43 705 A1 both of which are relied on forthis purpose and incorporated herein by reference.

In accordance with EP 423 627 B1, salts of the appropriate metals arereacted individually or as a mixture with tetraalkyl ammoniumtriorganohydroborates without use of hydrogen in tetrahydrofuran (THF)to prepare alloy colloids. The alloy colloids formed can be separatedfrom the reaction solution by filtration and can be very easilyredispersed in inert, organic solvents such as THF, diglyme orhydrocarbons.

DE 44 43 705 A1 describes the preparation of water-soluble,surfactant-stabilized alloy powders. For this purpose, metal salts arereacted in the presence of strongly hydrophilic surfactants from theseries of amphiphilic betaines, cationic surfactants, fatty alcoholpolyglycol ethers, polyoxyethylene carbohydrate fatty alkyl estersand/or anionic surfactants and/or amphiphilic sugar surfactants in THF,alcohols or directly in water with chemical reducing agents such ashydrides, hydrogen or alkali formiate between 0 and 100° C. and thenisolated. The alloy colloids so obtained are water-soluble. For purposesof the instant invention, amphiphilic betaines such as3-(N,N-dimethyldodecylammonio) propane sulphonate, lauryldimethylcarboxymethyl ammonium betaine or cocoamidopropyl betaine are preferablyused on account of their ready decomposability through temperaturetreatment.

After impregnation, the product obtained is washed with appropriatesolvents. The stabilizing surfactant shell of the colloid particles isremoved by calcination, which simultaneously activates the catalyst forits use as anode catalyst in the PEM fuel cell.

In contrast to the Pt/Ru alloy catalysts produced in conventional mannerby calcination at high temperatures, the supported preformed colloidsnot only display an improved precious metal dispersion and smaller alloyparticles, but they are also distinguished as having a very good COtolerance as a result of the higher accessible precious metal surface.

In the case of conventional alloy catalysts of the two metals, thetemperature needed for the alloy formation leads to a pronouncedcoarsening of the precious metal particles and to an exchange of thelattice sites of platinum and ruthenium. As a result, part of theruthenium is no longer available at the particle surface. Both effectslead to a decrease in the performance data under reformate/airconditions with concentrations of carbon monoxide in the anode gasgreater than 100 ppm.

Various measures in the preparation of the Pt/Ru colloid catalysts ofthe invention ensure that the bimetallic colloid particles are presenton the carrier in high dispersion and that no coarsening of thepreformed alloy particles occurs.

The temperature during the precipitation of the colloid particles isthus maintained between 20 and 110° C. depending on the solvent used anda highly dispersed distribution of the bimetallic colloid particles isachieved by optimizing the speed of addition, reducing the diffusionlimitation by stirring and using electrolytes. After removing thesolvent by filtration or distillation, the catalyst is appropriatelydried under vacuum. Calcination of the catalyst at temperatures between200 and 400° C. under inert gas removes still adhering protectivecolloid without residue and activates the catalyst.

The catalyst is used to prepare various components of fuel cells. Thus,for example, it is possible to prepare gas diffusion electrodes thatcontain a porous catalyst layer on a hydrophobized, electricallyconductive substrate material. For this purpose, the catalyst isprocessed into an ink using a solution of NAFION® and applied in thisform to a conductive hydrophobized carbon paper (manufacturer: TORAY,TGC 90). The coating of the carbon paper with precious metal isconventionally 0.01 to 0.5, 0.2 mg precious metal/cm² being preferred.

Using a gas diffusion electrode it is possible to prepare a so-calledmembrane electrode assembly that contains a proton-conducting polymermembrane and gas diffusion electrodes on the anode side and on thecathode side, the above-described gas diffusion electrode being used forthe anode side.

As an alternative component for fuel cells it is possible to prepare aproton-conducting polymer membrane that displays one catalyst layer oneach of the anode side and the cathode side, the catalyst layer of theanode side containing the catalyst of the invention in theconcentrations already cited above.

The following examples serve for the better understanding of theinvention. The catalysts prepared in the Examples were characterizedusing X-ray diffraction (XRD) and transmission electron microscopy(TEM). They were then processed into a gas diffusion electrode and amembrane electrode assembly (MEA), the catalysts being used on the anodeside of the MEU.

The CO tolerance was determined in a PEM fuel cell with a cell format of25 cm². The fuel gas used was a simulated methanol reformed gas of thecomposition 50 vol. % hydrogen, 10 vol.% nitrogen, 20 to 25 vol. %carbon dioxide with up to 150 ppm carbon monoxide and oxygenconstituents up to 5 vol. %. The voltage drop ΔU (mV) occurring afteraddition of a specific amount of carbon monoxide constitutes a measurefor the CO tolerance of the catalyst. The smaller this voltage drop, thebetter the CO tolerance of the catalyst. The catalysts of the inventiongenerally display ΔU values that are markedly better than the comparablevalues of the commercially available catalyst.

EXAMPLE 1

The following procedure was adopted to prepare a platinum/rutheniumcolloid catalyst according to the invention with a precious metalcontent of 20 wt. % and a platinum/ruthenium atomic ratio of 1:1.

To a solution of 56.5 g Pt/Ru colloid (Pt/Ru atomic ratio: 1:1,proportion of Pt in the colloid: 6.5 wt. %, prepared according to EP 423627 B1, Example 10, from PtCl₂ and RuCl₃ with tetrabutyl ammoniumtriethyl hydroborate in THF) in 2100 ml toluene under a current ofnitrogen there was added in each case 500 ml acetone and toluene and22.25 g Vulcan XC-72 (Cabot) was suspended therein with stirring. Thiswas initially stirred for 30 min at room temperature and then at 50° C.also for 30 min. The catalyst was filtered off, washed with 100 mltoluene/acetone (1:1) and dried at 80° C. in a vacuum. The catalyst wasthen calcinated at 350° C. for 1 hour under a current of nitrogen.

The XRD spectrum of the catalyst showed the shifted (110) reflex of theplatinum at about 40° (2 theta), suggesting an alloy formation withruthenium. In contrast, the (111) reflex of the ruthenium at 2 theta=44°could not be detected. The crystallite size of the platinum/rutheniumcrystallites was approximately 1.1 nm, the lattice constant was 0.388nm.

EXAMPLE 2

Another platinum/ruthenium colloid catalyst according to the inventionwas prepared as follows with a precious metal content of 20 wt. % and aplatinum/ruthenium atomic ratio of 1:1:

16.5 g Vulcan XC-72 (Cabot) were suspended in 1000 ml toluene andreacted with a solution of 40.0 g of the Pt/Ru colloid of Example 1 in1000 ml toluene at room temperature under a current of nitrogen. Thiswas initially heated to reflux for 60 min and the solvent removed in avacuum at 50° C. The catalyst was then washed with 100 ml acetone anddried in a vacuum at 80° C. The catalyst was subsequently calcined for 1h at 350° C. under a current of nitrogen.

The X-ray analysis of this catalyst also showed the presence of a Pt/Rualloy, the crystallite size was 1.8 nm, the lattice constant 0.388 nm.

COMPARATIVE EXAMPLE 1

For the following working examples, a commercially availablePt/Ru-supported catalyst (E-TEC) with a precious metal content of 20 wt.% and a Pt/Ru atomic ratio of 1:1 was also used. The X-ray analysis(XRD) of this catalyst also showed the presence of a Pt/Ru alloy as inExamples 1 and 2. The crystallite size (XRD) of the Pt/Ru crystallitewas, however, 2.7 nm.

WORKING EXAMPLE

The catalysts of the preceding examples were in each case processed toan ink using a solution of NAFION® and applied in this form to aconducting hydrophobized carbon paper (manufacturer: TORAY, TGC 90). Thecoating was in all cases 0.2 mg precious metal/cm². The anode preparedin this manner was hot-pressed together with an ion-conductive membrane(NAPION® 117) and a cathode electrode (coating 0.3 mg Pt/cm²) and amembrane electrode assembly (MEA) prepared in this manner.

The electrochemical performance data was measured in a single cell(pressureless operation, temperature 75° C.), a current density of 0.5A/cm² being set.

A fuel gas of the following composition was selected:

58 vol. % hydrogen, 15 vol. % nitrogen,

24 vol. % carbon dioxide, 3 vol. % oxygen.

The carbon monoxide content of the fuel gas was adjusted to 100 ppm andto 120 ppm in a second measuring series. The measured voltage drops ΔUcompared to the measurement without addition of carbon monoxide arelisted in Table 1.

                  TABLE 1                                                         ______________________________________                                                      100 ppm CO                                                                              120 ppm CO                                              Example  ΔU [mV]  ΔU [mV]                                       ______________________________________                                        B1            18        27                                                      B2          16              37                                                V1          80              128                                             ______________________________________                                    

The PEM fuel cell with the comparative catalyst VI shows the highestvoltage drop. The lowest voltage drop with high carbon monoxide contentsof the fuel gas is displayed by the PEM fuel cell with the catalystaccording to Example 1 which contained the smallest alloy particles(only 1.1 nm compared to 2.7 nm in the case of the comparativecatalyst).

Further variations and modifications of the foregoing will be apparentto those skilled in the art and are intended to be encompassed by theclaims appended hereto.

German priority application 197 56 880.7 is relied on and incorporatedherein by reference.

What is claimed is:
 1. A platinum/ruthenium alloy catalyst comprisingfinely dispersed alloy particles on a powdery, electrically conductivecarrier material, wherein the alloy particles have a mean crystallitesize ranging from 0.5 to less than 2 nm.
 2. The platinum/ruthenium alloycatalyst according to claim 1 wherein said means crystallite size rangesfrom 1.1 to 1.8 nm.
 3. The alloy catalyst according to claim 1, whereinthe electrically conductive carrier material is a member selected fromthe group consisting of carbon black, graphited carbon black, graphiteand active charcoal.
 4. The alloy catalyst according to claim 1, whereinthe ratio of platinum and ruthenium ranges from 10 to 50 wt. % relatedto the total weight of the alloy catalyst.
 5. The process according toclaim 4, further comprising stabilizing said alloy colloid with ahydrophilic surfactant selected from the group consisting of amphiphilicbetaines and using an aqueous solvent to apply the colloid to thecarrier material.
 6. A process for the preparation of an alloy catalystaccording to claim 1, comprising suspending an electrically conductivecarrier material in a solvent to form a suspension and depositing alloyparticles on said carrier material by adding a preformed,surfactant-stabilized, bimetallic platinum/ruthenium alloy colloid atelevated temperatures between 20 and 110° C., removing said solvent tothereby separate said catalyst from liquid phase of the suspension, andactivating said catalyst by calcinating at temperatures between 200 and400° C. under inert gas.
 7. The process according to claim 6, furthercomprising washing said catalyst prior to calcination.
 8. An alloycatalyst prepared by the process according to claim
 7. 9. The processaccording to claim 6, wherein said solvent is an organic solventselected from the group consisting of tetrahydrofuran, diglyme andhydrocarbons.
 10. An alloy catalyst prepared by the process according toclaim
 6. 11. A gas diffusion electrode for the anode side of a PEM fuelcell that comprises a porous catalyst layer on a hydrophobized,electrically conductive substrate material, wherein the catalyst layercontains the bimetallic alloy catalyst according to claim
 1. 12. Amembrane electrode assembly for PEM fuel cells that comprises aproton-conducting polymer membrane and gas diffusion electrodes on theanode side and on the cathode side, wherein said gas diffusion electrodeis the gas diffusion electrode according to claim 11 on the anode side.13. A proton-conducting polymer membrane for PEM fuel cells whichdisplays one catalyst layer each on the anode side and on the cathodeside, wherein the catalyst layer on the anode side contains the catalystaccording to claim
 1. 14. A PEM fuel cell comprising a catalystaccording to claim 1.